1. Field of the Invention
The present invention relates to an ultrasonic microscope having a focusing mechanism and using a cryogenic fluid as a medium for transmission of an ultrasonic beam.
2. Description of the Related Art
Japanese Patent Publication No. 59-44582 and Japanese Patent Disclosure No. 58-196453 disclose ultrasonic microscopes in which a sample is two-dimensionally scanned with an ultrasonic beam, and the waves passing through, or reflected from, the sample are processed to form an image of the sample.
In these microscopes, the ultrasonic beam must be focused onto the sample in order to form a clear image of the sample. To achieve a successful focusing of the beam, it is necessary to adjust, with high accuracy, the distance between beam-focusing means, e.g., an acoustic lens, and a surface or an internal plane of the sample, thus setting this distance within the focal depth of the microscope. The Journal of Acoustic Society of America, Vol. 67 (1980), pp. 1629-37, discloses a cryogenic ultrasonic microscope which can provide a high-resolution ultrasonic image of a sample. To conduct the ultrasonic beam from the acoustic lens to the sample efficiently, a beam-transmitting medium is filled in the gap between the acoustic lens and the sample. The beam transmitting medium is cryogenic liquid such as liquid nitrogen, liquid argon, or liquid helium, which transmits sound more slowly and absorbs less than water.
The cryogenic ultrasonic microscope also has a focusing mechanism. The mechanism is located outside the housing containing the cryogenic liquid. Due to the position of the focusing mechanism, the microscope as a whole is complex in structure, and heat can easily propagate into the housing. Hence, the temperature of the cryogenic liquid is likely to rise, making it impossible to accomplish high-accuracy focusing of the ultrasonic beam.